The present invention relates generally to the operation of small, low-cost DC motors, and more particularly to sensing and switching circuits needed to sequentially power the coils of the motor in synchronism with the movement of the rotor.
Small, low-cost dc motors are used in a large number of low-power and battery operated equipment. For example, such DC motors are often used as motors for continuous operation fans. In such applications the reliability and safety of the equipment is dependent upon the performance and reliability of the fan motor. Such motors are therefore key to the continuous availability of the equipment. An example is a fan in a personal computer. A fan in the body of the computer carries out an essential function by cooling the main processor unit, thereby protecting it from damage caused by overheating. The costs of personal computers have been subject to considerable price erosion, and a low-cost, but highly reliable, fan drive circuit is an essential part of such equipment.
DC motor drives typically contain a magnetic sensitive device for detecting the position of a permanent magnet rotor of the motor, and switching circuitry to switch the voltage from coil to coil to maintain the rotation. However, additional circuit components must usually be present to reduce the radio frequency interference, RFI, generated by the switching process so as to protect the switching and measuring circuits from transients and interference. Further, additional circuitry must be present to supply a suitable voltage power supply to the measurement circuitry and to protect against a reversal of the power supply feed and additional Electro-static discharges, ESD, associated with static build up and handling hazards.
According to the invention, a DC motor drive circuit includes a switching circuit that drives the coils of a small permanent magnet DC motor. The drive circuit is preferably implemented in an integrated circuit device, such as a silicon CMOS device. Integrated into the same integrated circuit device is a magnetic sensor arranged to detect the position of the permanent magnet as it passes a defined point, or points, in its revolution, and control circuitry to derive the timing waveforms for driving the coils. The integrated power devices that drive the coils are also arranged to limit the rise and fall times of the applied voltages and currents so as to reduce or eliminate the generation of unwanted RFI.
Additional circuitry is also integrated into the same integrated circuit device to derive the necessary power to operate the magnetic sensor, the control circuitry and the switching circuitry from the connections between the switching circuitry and the coils so as to remove the need for a separate power supply connection. Deriving the power necessary to operate the electronic circuitry in this manner has the additional benefit that the inherent inductance and capacitance of the coils and motor acts both as a natural suppression for ESD transients and also as a current limitation device in the event of inadvertent power supply reversal or transient surge in the power supply which would normally cause destructive damage. The output drivers are also preferably protected by zener diodes to add surge and transient protection. The integrated circuit design takes advantage of these inherent benefits and includes circuit elements designed to ensure that expected ESD transients and reverse and over voltage transient power supply connections can be survived without damage to the device or deterioration in its performance.
In one embodiment of the invention, the magnetic detection element includes a Hall effect device, and two coils that are used to drive the motor. In this embodiment, the complete switching and control circuit needs only three connections and can be mounted in an industry standard, low-cost three-pin package.
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.